This invention relates, in general, to the manufacture of semiconductor wafers, and more particularly, to reducing autodoping during the growing of an epitaxial layer on a semiconductor wafer.
Autodoping has been a persistent problem to the deposition of epitaxial layers on a semiconductor substrate. This is particularly true for a low resistivity semiconductor substrate. Autodoping is a result of two distinct occurrences; one is that caused by outdiffusion from the low resistivity substrate into the epitaxial layer, and the other is that caused by vaporization of the impurities in the substrate which enter into a gaseous phase of the epitaxial layer being deposited. The autodoping affect produces an epitaxial layer having non-uniform impurity concentrations. Therefore, it is difficult if not impossible to accurately control the impurity concentration within the epitaxial layer because of the autodoping affects. Autodoping can cause a semiconductor wafer to be a low yielding wafer.
Since antimony outdiffuses less than arsenic, boron, or phosphorous, many semiconductor wafer manufacturers use antimony as the impurity dopant in the semiconductor wafer. However, some semiconductor devices require low resistivity substrates and therefore the semiconductor manufacturer is left with no choice but to use phosphorous or arsenic. Since autodoping can also result from vaporization of impurities from the backside of the wafer, it is generally customary to seal the backside of the wafer with a backseal such as oxide or nitride. There is not believed to be an effective front seal for the front or top of the wafer.
Autodoping from a highly doped diffused region in a substrate during growth of an epitaxial layer has been reduced by the growing of a thin epitaxial layer over the entire surface of the substrate and then removing the epitaxial layer except for the portion over the highly doped diffused region. Typically this thin epitaxial portion then becomes part of the subsequently grown epitaxial layer, the substrate is of a different conductivity type than the small portion of thin epitaxial layer, and such autodoping minimization is believed to have only been used for integrated circuits. Examples of this technique can be found in U.S. Pat. No. 3,660,180 which issued to Wajda, and in U.S. Pat. No. 3,716,422 which issued to Ing et al. Minimizing autodoping is also discussed in IBM Technical Disclosure Bulletin, Vol. 14., No. 11, April 1972, page 3218; IBM Technical Disclosure Bulletin, Vol. 15, No. 11, April 1973, page 3385; and in IBM Technical Disclosure Bulletin, Vol. 20, No. 3, August 1977, pages 1083-1084. However, it would be desirable to provide a front seal for an entire wafer and not just for a localized highly doped region.
Accordingly, it is an object of the present invention to minimize autodoping from a low resistivity semiconductor wafer.
Another object of the present invention is to minimize autodoping from a low resistivity wafer during the growing of an epitaxial layer over the wafer by using a thin epitaxial layer of the same conductivity as the wafer and which serves as a seal.
Yet a further object of the present invention is to provide an epitaxial front seal for a low resistivity N-type substrate so that aluminum can be used as a back metal.